The present invention generally relates to a detection system. More particularly, the invention relates to a detection system by which the presence or absence and type of fluid unintentionally or accidentally present in confined areas, such as within an underground storage tank system may be identified.
In certain systems, the presence of fluids is unwanted and may be indicative of a more serious problem. Underground storage tank systems include confined areas in which the presence of fluid may be indicative of a serious problem.
An underground storage tank is any tank that has at least a portion of its volume positioned below ground level. The tanks are largely made from metal, such as steel with or without coating, or synthetic materials, such as plastic or fiberglass. One type of underground storage tank, the double-walled underground storage tank, is constructed such that an inner wall retains the stored fluid or substance and a surrounding, but separate, outer wall acts to confine any fluid that leaks from the inner wall and to maintain the integrity of the inner wall. It is into the interstitial space that opens between these walls that ground water from the outside may enter or that leaked fluid from the inner wall may enter, such as after a portion of the wall has corroded away or has been disrupted by the shifting of the underlying or surrounding soil or by unsupervised digging.
An underground storage tank is one part of a system that includes piping by which the fluid or substance to be stored in the tank is delivered to the tank and by which the same fluid or substance is drawn from the tank, such as with the use of a dispenser. Any one location may have a plurality of such tank systems. An example of such a location having a plurality of tank systems is a gasoline service station in which products, such as different grades of gasoline may be stored in and dispensed from different underground storage tanks. Dispensers at such a location consist of conventional gasoline pumps. In order to protect the tank system piping from disruptive effects, and to prevent spills--that occur during the filling of or dispensing from the tanks--and leakage from the piping from entering the surrounding environment, underground tank storage systems may include one or more containment chambers. Such chambers generally surround, for example, the area at which products are delivered to the tank and the areas from which the products are dispensed from the tank.
Underground storage tanks, particularly those that are buried completely below ground, are the preferred means to store and dispense fluids--such as gasoline, oils, petroleum products, chemicals--that are toxic and/or hazardous. Underground storage tanks are proportionately less exposed and thereby better protected from accidental rupture. Furthermore, if such tanks do leak or spills do occur, the fluid is accepted into the surrounding soil thereby preventing immediate exposure to the public. Furthermore, placing the tank underground saves above ground space for other purposes.
Given the need for and advantages of underground storage tank systems, it is not surprising that there are an estimated several million such systems currently in use throughout the United States. One of the largest uses of underground storage tanks is for the storage of gasoline and related fuel products at services stations across the country.
One serious consequence that occasionally results from the use of underground storage tanks, however is the emission of product when it is being delivered to, stored within, and/or drawn from the tank. The product as stored within the tank may be leaked into the environment because of corrosion, disruption, and/or faulty installation of the tank, the system piping, and/or the dispenser. Underscoring the leakage problem is that it is estimated that some twenty-five percent of all underground storage tanks are said to be now leaking.
Substances leaking from tanks can contaminate the surrounding soil, air, and ground water. Contamination of the ground water supply is serious since some fifty percent of the U.S. population still depends on ground water as a source of drinking water and because ground water is a resource that is renewed at a particularly slow rate. Depending upon the type of fluid or substance that is leaked, poisonous and/or explosive vapors may result. These vapors tend to accumulate in confined spaces such as basements, septic tanks, or sewers causing hazardous and possibly life threatening situations. Tank leaks are costly not only because of the loss of the stored product and because of the fines that may be imposed under federal, state, and/or local laws, but also because ultimately the released product must be cleaned up. Clean-ups often cost $100,000. or more.
A variety of devices and methods by which leaks from underground storage tanks may be detected are known. However, a variety of disadvantages are associated with them. For example, double-wall underground storage tanks made of a metal or coated metal conventionally include a monitoring pipe extending vertically adjacent to and generally approximate to one or more of the corners of the outer wall of the tank. A section of pipe opens between the monitoring pipe and the tank such that any fluid that leaks onto the inner basal area of the interstitial space--either because of a breach of the outside wall or a breach of the inner wall of the tank--may drain toward and onto the floor of the monitoring pipe. Whether a leak has occurred from a tank containing gasoline or other volatile fluid is determinable from the vapors that will be emitted. The vapors are detectable upon removal of the surface cap of the monitoring pipe. The presence of non-volatile fluids in the monitoring pipe, such as water, is conventionally detected by lowering and retrieving a sampling device down the length of the monitoring pipe. Such a sampling device may be as simple as a long cloth rag. Similar known methods and devices are used to detect the presence of leaked or spilled fluids in containment chambers. One of the many problems associated with such monitoring devices and methods is that they require personal attention and do not provide continuous monitoring. Without continuous monitoring, a leak may go undetected for a long period of time thereby allowing what may be a minor problem to become exacerbated.
Other conventional systems signal the presence of fluid in the detection area by the movement, for example, of a probe. Such a probe system may include a float that signals that a leak has occurred upon the float being buoyed up by the liquid.
Systems that utilize moving parts, such as floats, are particularly problematic in that they do not identify the presence of a leak until a sufficient amount of liquid has collected so the float can be buoyed up. If the fluid does not collect in such a way as to buoy up the float, the signal is not triggered. Furthermore, the float's ability to become buoyant depends not only on the material characteristics of the float and its own weight, but also the weight of that which is attached to the float. Very often power lines and/or guide wires are attached to the float. These elements act to weigh the float down. Floats can also bind, get caught, hang up, or lose their buoyancy with time, thereby decreasing the ability of the system to identify that a leak is occurring.
Other conventional systems utilize certain means to detect the presence of a fluid chemically or electrically. These known probes are disadvantageous in that they have a generally limited period of usefulness and/or cannot be reused. For example, some such probes utilize exposed metal surfaces to sense the presence of a fluid. However, the effectiveness of such probes largely decreases with time and exposure because the metal surface oxidizes and/or becomes covered with film or dirt. Other probes utilize a reactive face that changes in response to a liquid such as by undergoing an irreversible chemical reaction. Such probes must be replaced after contact with a liquid is made.
Many known detection systems are limited in their applications. For example, some systems can be positioned in the interstitial area between the walls of a double walled tank system, or in a monitoring pipe, or in a containment chamber around piping or under a dispenser, but not necessarily in all of these applications. Other systems are capable of detecting one or a very limited number of fluids but are generally not tunable to selectively detect one or more of a wide range of fluids.
Other known systems that do utilize some type of circuitry to identify leaks tie the electrically-powered probe to a control assembly by conventional means, such as lengths of wire. Such systems are generally not reliable because the noise that may be generated over a length of the wire is not filtered out thereby giving rise to spurious signals.
A demand therefore exists for a system and methods with which the presence or absence of fluid within, for example, the interstitial space between the inner wall and outer wall of an underground storage tank, within the monitoring pipe adjacent to an underground storage tank, and/or within a containment chamber--such as which surrounds the piping leading to or from an underground storage tank or that is below and surrounds the piping to a dispenser--may be monitored reliably and on a continuous, unattended basis. The present invention satisfies this demand.
The fluid detection system of the present invention uses the capacitance that develops across an unique comb-like electrode assembly within a unique liquid sensing probe to identify the presence and/or absence of a fluid and, more specifically, the type of the fluid that may enter a confined area within an underground storage tank system. It is known that the capacitance between two closely spaced, but separate, plates will also be affected by the distance between the plates: the greater the distance between the plates, the lower the capacitance developed across the circuit. Capacitance will be affected also by the material--termed the dielectric--present between the plates. Each dielectric is assigned a dielectric constant that is indicative of the ability of the dielectric to affect the capacitance generated across a circuit. Air has a dielectric constant of one. The system of the present invention utilizes the particular dielectric constant, or signature, that each fluid has to identify the presence or absence of and the type of fluid that is encountered by the electrode assembly in the system.
More particularly, a detection system according to the present invention includes a assembly designed to monitor conditions within and to detect the presence and/or absence and types of fluids in confined areas in which inspection, visually or otherwise, is generally impossible or difficult to accomplish on a regular and/or cost effective basis. Such confined areas include those areas within underground storage tank systems where fluids unintentionally or accidentally entering the system can accumulate, such as the floor of a monitoring pipe, the floor of a containment chamber, or the inner basal area of the interstitial space between the walls of a double-walled tank. The probe includes a sensor that is positionable approximate to, but separate from an accumulation area. Within the probe, a sensor having specialized solid state liquid sensing circuitry and, in particular, a electrode assembly across which, in response to contact with a fluid or substance present in an accumulation area, capacitance develops that is characteristic of the fluid or substance. The system circuitry responds to the capacitance of the electrode assembly by generating a variable frequency signal for transmission to a surface-mounted, analyzer unit. The liquid sensing circuitry further includes within the analyzer unit, logic by which frequencies characteristic of humidity and/or temperature ambient conditions, and frequencies characteristics of fluids not of interest, are removed from the output signal. The edited output signal is digitized and transmitted to a central display location, located for example, above ground and remote from the underground storage tank system.
The detection system of the present invention may be advantageously used in a variety of applications. For example, the probe assembly may be lowered into the interstitial space between the walls of a double-walled underground storage tank, lowered into monitoring pipes, such as those typically associated with metal or metal underground storage tanks, or positioned within the containment chambers that are approximate to the filling area and/or dispensing area of such tanks.
The detection system utilizes no moving parts, such as a float, that can hang up or otherwise provide a false reading. Instead, the probe assembly is capable of identifying the presence or absence of fluids by a specialized sensor circuitry. The detection system does not require a great amount of fluid to accumulate before the presence of the fluid is identified. A liquid film accumulation as shallow as one thirty-second of an inch may be identified by the detection system.
The sensor of the present invention has no elements that are exposed or otherwise can be altered, especially in the presence of the fluids to be monitored. The probe assembly detects without the destruction of the sensor thereby obviating the need to replace the sensor after a single contact with fluid.
The sensor circuitry of the probe assembly furthermore may be tuned to the capacitance generated by water and any one of a variety of fluids, including gasoline, petroleum, diesel fuel, or methanol.
The sensor is advantageously positionable by the probe assembly approximate to but separate from the accumulation area to prevent the system from providing the false readings that intimate contact with the accumulation area would provide.
The probe assembly of the present invention provides a variable frequency output signal. A signal provided in this form is less subject to interference by customary occurrences, such as adjacent power lines or variations in power line voltage.
The variable frequency output signal generated by the probe assembly of the system is analyzed by an "intelligent" frequency counter which reads the signal on a constant basis and separates frequencies, such as that generated by "noise", fluids of no interest, and that generated in response to ambient moisture and temperature conditions.
The resulting output signal is digitized to allow the signal to be reliably transmitted over long distances without risk of electrical interference. The signal may be displayed at a location, such as a ground level display, and/or at a remote location such that the conditions within a multiple of accumulation areas may be displayed at once.
It is, accordingly, a general object of the present invention to provide a system by which conditions within underground storage tank systems may be monitored.
Another object of the present invention is to provide a system that is capable of detecting the presence and/or absence of a wide range of fluids, including water.
It is an additional object of the present invention to provide a detection system by which one of a variety of fluids may be separately identified within an accumulation area of an underground storage tank system.
It is a further object of the present invention to provide a system that includes a sensor that contains no exposed metal surface or reactive surface and that is not prone to oxidation and/or corrosion or otherwise is alterable in response to the presence of a fluid.
A further object of the present invention is to provide a system that includes a sensor by which the presence or absence of and identity of a wide range of fluids may be detected reliably and continuously.
Also an object of the present invention to provide a system that includes a sensor that can detect a wide range of fluids without the destruction of and need for the replacement of the sensor.
Another object of the present invention is to provide a system that includes circuitry by which signal noise, such that generated by ambient conditions and non-selected fluids may be filtered out.
Additionally, an object of the present invention is to provide a system that includes a transmitter by which the signal may be digitized for transmission without a loss in information.
These and other objects, features, and advantages of this invention will be clearly understood and explained with reference to the accompanying drawings and through a consideration of the following detailed description of the preferred embodiments.